Some I haven't seen mentioned:
Find iPhone - absolute must for iPhone owners, locate iPhone on map, remotely lock iPhone, remotely wipe iPhone, make iPhone play siren even if its on silent
Tweetbot - my favorite twitter client
Deliveries - easy way to track any packages you have through UPS, USPS, FedEx, DHL etc..
Spotify - $10/mo, but can listen to any song you want on demand, and share playlists with your friends
RedBox - reserve movies right from your phone to go by and pick up
Evernote - note-taking app that can be synced across pretty much any device you may have. supports attachments
TripIt - easy way to keep up with your trip itinerary just by forwarding your hotel/flight/car confirmation emails
mSecure - keep up with all of your online passwords and such
Sleep Cycle - app that wakes you up when you are in a light sleep as opposed to deep REM sleep (detects your movements throughout the night)
Songkick - will notify you when artists in your iTunes library are playing concerts near you
Soundtracking - post clips of the songs you listen to on twitter/facebook/email
Crackle - free movies and tv shows
If you're lucky enough to have Verizon Wireless:
NFL Mobile - watch NFL Redzone live all day on Sunday, plus live Sunday and Monday night footlball, also scores, stats, and highlights
VZ Contact transfer - keep your contacts backed up
My Verizon Mobile - everything you need to stay on top of your online account